How do I fix a thermometer if the mercury has separated?

June 27, 2008 9:46 AM

How do I fix a thermometer when the mercury has separated?

I purchased a freezer thermometer. After getting it home the mercury (or whatever liquid they use in them now) had separated near the top of the scale, so there are bands of air between bands of red.

I tried heating it up, but this just compresses the band separation, then it returns to normal.

Instructions online say the fix is to get it cold enough that all of the liquid is pulled into the bulb. This thermometer goes to at least -35F on the readable scale, and a little past that before it reaches the bulb, probably -50F from the looks of it.

How can I fix it? Alternatively - do you work with liquid nitrogen and would you like to fix my thermometer?

posted by odinsdream to science & nature (14 comments total)

If it's red then it's alcohol and not mercury (which is silver), but not sure how you'd fix it I'm afraid. Centrifuge ?
posted by zeoslap at 9:57 AM on June 27, 2008

I should have added that my centrifuge is currently out of order.
posted by odinsdream at 10:01 AM on June 27, 2008 [2 favorites has favorites]

Have you tried shaking it down? Also try putting it in the coldest part of the freezer, right where the air comes out.
posted by TedW at 10:02 AM on June 27, 2008

You can use the principles of an ice cream maker -- ice and salt -- to get a low temperature without resorting to liquid nitrogen or some such exotic material. Or perhaps dry ice in a sealed container.
posted by Cool Papa Bell at 10:05 AM on June 27, 2008

Urgh broken centrifuge and thermometer, it's not your day is it :) How about attaching it to an electric drill with a string, and using that as a centrifuge... It's crazy enough that it might just work (and failing that you'd be a hit on youtube if you got it on video)
posted by zeoslap at 10:06 AM on June 27, 2008 [1 favorite has favorites]

uh, you'd best let that dry ice vent.... unless you *are* trying to be on a youtube video.
posted by nat at 10:19 AM on June 27, 2008

You're not going to get it cold enough with ice/salt (only goes to about -10 deg C).

You're going to need some dry ice/solvent mix (isopropanol is easiest to find, but most common organic solvents would work). Dry ice is ridiculously cheap and if you asked a grad student nicely you could probably just use a little from an area biology or chemistry department. Just pour a little solvent over some dry ice in a container. Once the bubbling stops, as long as there's solid dry ice still visible, the liquid is at about -78 deg C. Stick the thermometer in there, tap it a bit to help the bubble escape, and try to let it warm it up slowly (or you can make matters worse).

Liquid nitrogen does work too, but also leads to more frequent shattering (a lesson I keep relearning the hard way).

But in all honesty, you should be able to just exchange it. A new thermometer shouldn't come that way.
posted by Dr.Enormous at 10:53 AM on June 27, 2008 [2 favorites has favorites]

Should have mentioned I tried the ice-salt bath, but I really wasn't expecting much, since it's nowhere near the lower range of the thermometer. Compressed air maybe? Come on ideal-gas law!
posted by odinsdream at 10:58 AM on June 27, 2008

Also tried shaking it. Vigorously. With apparently no change at all.
posted by odinsdream at 11:00 AM on June 27, 2008

You just bought it? Take it back to the store and get it replaced, its not fit for purpose.
posted by missmagenta at 11:00 AM on June 27, 2008

Seconding that you ought to return it, but if that's not possible, i would try again heating it. There's no air in there, or very little. If there were, the pressure would not allow the liquid to rise. So there's no air in between the sections that compresses. Try heating it up very slowly in water and see if you can't get the sections to rejoin that way. The other option is, instead of just shaking, actually thump the thing hard onto a solid surface, making sure you don't dislodge and break the tube (maybe put some tape on it first). Do a few thumps and see if anything moves.
posted by beagle at 11:18 AM on June 27, 2008

I've done this: dry ice/acetone bath is the only way to go if you don't have access to liquid nitrogen. It shouldn't cost more than $20 and you'll have more than enough dry ice for a really good party afterwards.

Don't heat it or use an improvised centrifuge unless you want to practice with the mercury spill kit. Anyway, centrifuges work well with spirit thermometers, but I've never had much luck with mercury ones.

Why do this instead of buying a new one? It's a lot cheaper to spend $20 on a bag of dry ice than to buy a new, certified thermometer at $50 to $150.
posted by bonehead at 12:37 PM on June 27, 2008

bands of air between bands of red

Duh, scratch that: you've got a methanol spirit thermometer. Centrifuge is best, followed by freezing. Both will work, freezing costs more.

I presume exchanging is out of the question, for some reason.
posted by bonehead at 12:40 PM on June 27, 2008

beagle has it! While I would have loved to play with chilling the thermometer to -60F, I tried heating it again but was much more careful this time, using a small bowl of hot water and carefully dipping to slowly increase the temperature up to almost the maximum point, then doing the reverse to cool it down until it reached room temperature without separating. All fixed. Thanks everyone!
posted by odinsdream at 5:17 PM on June 27, 2008